1. Field of the Invention
This invention relates to an automatic adjustment circuit for an analog control unit which electronically performs automatic control of gain errors which occur in an analog control unit such as a video intermediate frequency amplification circuit (VIF circuit) or a sound intermediate frequency amplification circuit (SIF circuit) provided in an electronic apparatus such as a television receiver (TV).
2. Prior Art
In a VIF circuit of a TV set for example, an automatic gain control circuit (AGC circuit) is provided which supplies negative feedback of voltage to a TV tuner so that a detection output remains constant even where an input level received by a TV antenna varies. However, TV tuners and VIF circuits have their own gain errors and errors also occur in the AGC voltage. Conventionally, these gain errors were adjusted by changing an adjustment point of the AGC circuit by a semifixed resistor 24 provided in the VIF circuit 2 as shown in FIG. 9. Ordinarily, this adjustment was performed by adjusting the semifixed resistor 24 on the TV production line either manually by a worker or mechanically using a factory automation apparatus.
However, this adjustment is one of very intricate processes to be performed on the production line, and besides the preciseness of adjustment is not fully reliable, so that said adjustment is not preferable for mass production of good products.
Said adjustment is required not only for TV VIF circuits but also for other blocks of a TV such as SIF circuits, stereo circuits, character broadcast receiving circuits as well as for various types of analog control units used for other household and office appliances. Accordingly there is a great demand for a adjustment circuit which can automatically adjust for the errors of analog control units in a more simple and efficient manner.